Resilient Modulus Test — Triaxial Cell Interaction

This paper describes how, during the development work by the Strategic Highway Research Program (SHRP) from 1987 through 1992 and the Federal Highway Administration (FHWA) from 1993 until the present, it was recommended that knowledge and assessment of triaxial equipment and sample interaction be correctly applied to both test command and data reduction routines for the accurate measurement of resilient modulus properties of unbound materials using external instrumentation configurations. This paper presents the sensitivities of the results because of appropriate and inappropriate interpretation of the correction factors necessary, as well as the influences of seal drag forces. The sensitivities have been calculated for two specific triaxial cells. Note that each triaxial cell and instrumentation configuration possesses unique values that are needed for the correction. Generally, the larger the triaxial cell, (larger rod diameter, large mass of rod), the greater the influence of the uplift and static weight components of the load. The paper describes how users must be knowledgeable about their test equipment and the ability of the software to correctly apply proper loads and properly assess these loads for the correct calculation of resilient modulus. Further, physical tolerances must be developed for triaxial cells with respect to seal drag, alignment and compliance. These issues are extremely important when comparing results between laboratories, as the errors associated with miscalculation, misinterpretation or incorrect measurement of loads can result in variations outside the precision of the test itself.